The present invention relates to a novel process for the preparation of Lithium hexafluoro phosphate (LiPF6). More particularly, the present invention relates to a novel process for the preparation of Lithium hexafluoro phosphate (LiPF6) which is useful as a battery electrolyte especially for rocking chair cells and other primary non-aqueous lithium based cells and solid state and solid polymer electrolyte lithium cells.
LiPF6 is a well-known electrolyte for secondary lithium based cells which is difficult to prepare because of very active raw materials like P2O5, Li2O and PF5 and F2 gases. There are a few chemical procedures available in literature however, they need careful purification and handling. Moreover, such procedures require a wide range of equipment to operate and prepare this essential chemical for rocking chair or lithium ion or intercalation battery systems.
Metal fluorides of phosphorous viz. MPF6 where M is Ca, K or Li can be prepared by reacting MF with PF3. (Paul et. al., Fluorine Chemistry, 56, 1995). Hence,
3MF+5PF3xe2x86x923MPF6+2P
Where M is Ca, K or Cs MPF6 can be prepared by neutralising HPF6 with appropriate base viz.,
2HPF6+Ca(OH)2xe2x86x92Ca(MF6)2+2H2O
Similarly some of the hexafluorophosphates can be prepared by double decomposition of NH4PF6 or KPF6. (Paul et. al., Fluorine Chemistry, 56, 1995; Fluorine Chemistry, Vol. 5, Simons J. H., 131(1819), 1964; Fluorine Chemistry, Vol. 1, Emelson H. J., 76(219), 1995) LiPF6 prepared by earlier methods suffer has certain disadvantages like, they need toxic chemicals as reactants, it becomes difficult to handle the chemicals involved, the final product contains unreacted impurities, it is a partial reaction, it needs further steps for preparation and it is not a pure solid state reaction.
The main object of this present invention is to provide a novel method for the synthesis of (LiPF6) lithiumhexafluorophosphate as a battery electrolyte.
Another object of this present invention is to provide a complete solid state reaction.
Still another object of this present invention is to avoid many active chemicals which are difficult to handle.
Yet another object of this invention is to avoid obnoxious gases.
Still yet another object of this invention is to get the product without any partial reaction.
These and other objects of the invention are achieved and the problems associated with the prior art are overcome by the process of the invention described below.
A novel method for the preparation of LiPF6 (lithium hexafluoro phosphate) is developed where in equimolar quantities of pure dry AR lithium source such as Li2O or Li2CO3 or LiNO3 is mixed with (very pure dry and AR samples) diammonium hydrogen phosphate and the mixture was heated to 200xc2x0-600xc2x0 C. continuously for 6 hours in an electric furnace. The product was cooled and was then transferred into a dry vessel. The product was a transparent substance which was confirmed by X-ray analysis as LiAsF6.
Accordingly, the present invention provides a novel solid state thermal process for the synthesis of lithium hexafluoro phosphate (LiPF6), which comprises of:
(a) mixing a lithium source selected from the group consisting of a lithium oxide, a lithium salt and a combination thereof with diammonium phosphate in equimolar proportions;
(b) heating the mixture in a furnace at a temperature in the range of 150xc2x0 C. to 600xc2x0 C. continuously to obtain LiPO3;
(c) cooling and powdering the LiPO3 
(d) mixing of the powdered product with six to nine times quantitatively of ammonium fluoride and then heating it at a temperature in the range of 150xc2x0 C. to 200xc2x0 C. using a furnace continuously for 4-6 hours in a closed Teflon container;
(e) transferring the product to a dry vessel.
In one embodiment of the invention, step (b) above is carried out for a time in the range of 4 to 6 hours.
In an embodiment of the present invention LiPO3 is mixed with NH4F in the ratio 1:6-9 to form LiPF6.
In another embodiment of the present invention, lithium meta phosphate (LiPO3) is mixed with ammonium fluoride in a ratio of 1:7 and the mixture heated to 200xc2x0 C. in an electric furnace for 6 hours continuously to get lithium hexafluoro phosphate.
In yet another embodiment of the present invention Li2CO3 or Li2O or LiNO3 is mixed with diammonium hydrogen phosphate and ammonium fluoride in the molar ratios 1:2:6-9
In another embodiment of the present invention Li2CO3 or Li2O or LiNO3 is mixed with diammonium hydrogen phosphate and ammonium fluoride in the ratio of 1:1:7.
In yet another embodiment of the present invention, Li2O or LiNO3 is mixed with diammonium hydrogen phosphate and ammonium fluoride in the ratio 1:1:7 and the mixture is heated at 200xc2x0 C. continuously for 12 hours in an electric furnace to obtain LiPF6.
In an embodiment of the present invention Li2CO3 or Li2O or LiNO3 is mixed with diammonium phosphate in the ratio 1:1 and then, the product is mixed with NH4F in the ratio of 1:7 and heated.
In still another embodiment of this invention the temperature is kept between 150-350xc2x0 C. for the preparation of LiPF6 when LiNO3 or Li2O in the starting material.
In another embodiment of the invention, for Li2CO3 the temperature of heating with (NH4)2HPO4 is around 600xc2x0 C. and subsequently with NH4F mixed and heating the temperature is kept in between 150-350xc2x0 C.
In another embodiment of the invention, the reactions occur in solid state.
In yet another embodiment of the invention, the process comprises a two step reaction as follows:                     (        a        )                                                      2            ⁢                                          (                                  NH                  4                                )                            2                        ⁢                          HPO              4                                +                                    Li              2                        ⁢            O                          ⁢                  →                                    300              ⁢              xc2x0              ⁢                              xe2x80x83                            ⁢                              C                .                                      ⁢                          xe2x80x83                                      ⁢                              2            ⁢                          xe2x80x83                        ⁢                          LiPO              3                                +                      4            ⁢                          NH              3                                +                      3            ⁢                          H              2                        ⁢            O                                                                        2            ⁢                                          (                                  NH                  4                                )                            2                        ⁢                          HPO              4                                +                                    Li              2                        ⁢                          CO              3                                      ⁢                  →                                    600              ⁢              xc2x0              ⁢                              xe2x80x83                            ⁢                              C                .                                      ⁢                          xe2x80x83                                      ⁢                              2            ⁢                          xe2x80x83                        ⁢                          LiPO              3                                +                      4            ⁢                          NH              3                                +                      CO            2                    +                      3            ⁢                          H              2                        ⁢            O                                                                        2            ⁢                                          (                                  NH                  4                                )                            2                        ⁢                          HPO              4                                +                      2            ⁢                          LiNO              3                                      ⁢                  →                                    300              ⁢              xc2x0              ⁢                              xe2x80x83                            ⁢                              C                .                                      ⁢                          xe2x80x83                                      ⁢                              2            ⁢                          xe2x80x83                        ⁢                          LiPO              3                                +                      4            ⁢                          NH              3                                +                      2            ⁢                          NO              2                                +                      2            ⁢                          H              2                        ⁢            O                    +                      O            2                                                                                      (              b              )                        ⁢                          xe2x80x83                        ⁢                          LiPO              3                                +                      6            ⁢                          NH              4                        ⁢            F                          ⁢                  →                                     greater than                               150                ⁢                –300xc2x0                ⁢                                  xe2x80x83                                ⁢                                  C                  .                                                      ⁢                          xe2x80x83                                      ⁢                              LiPF            6                    +                      6            ⁢                          NH              3                                +                      3            ⁢                          H              2                        ⁢            O                              
In yet another embodiment of this invention a single step process is possible at a temperature greater than 150-300xc2x0 C.
Li2O+2 (NH4)2HPO4+12 NH4Fxe2x86x922 LiPF6+16NH3+9H2O
Li2CO3+2 (NH4)2HPO4+12 NH4Fxe2x86x922 LiPF6+16NH3+CO2+9H2O
LiNO3+2 (NH4)2HPO4+12 NH4Fxe2x86x922 LiPF6+16NH3+NO2+9H2O
In another embodiment of the present invention, pure dry Li2O or Li2CO3 or LiNO3 is mixed with twice the quantity of diammonium phosphate i.e., in the molar ratio of 1:2 and the mixture is heated in an electric or muffle furnace for 6 hours at a temperature of 600xc2x0 C. continuously for Li2CO3 and around 300xc2x0 C. for Li2O and LiNO3. The product was dried at 80xc2x0 C. and was then allowed to react with NH4F in the molar ratio of 1: greater than 6-8 for 4 hours in an electric furnace continuously at a temperature range of 150-350xc2x0 C.
In yet another embodiment of this invention all materials are in solid state conditions.
It is a feature of this invention that an electrical furnace can be used for the process.
It is another feature of this invention that a muffle furnace can be used for the process.
In yet another embodiment of this invention a single step process is possible viz.,                     (        a        )            ⁢              xe2x80x83            ⁢              LiPO        3              +          6      ⁢              NH        4            ⁢      F        ⁢      →                   greater than                   150          ⁢          –300xc2x0          ⁢                      xe2x80x83                    ⁢                      C            .                              ⁢              xe2x80x83              ⁢            LiPF      6        +          6      ⁢              NH        3              +          3      ⁢              H        2            ⁢      O      